Subsurface explorationists often employ seismic surveys to analyze subsurface structures. One particular type of seismic survey, Vertical Seismic Profiling (VSP), combines the use of downhole seismic sensors with seismic sources on the surface. Often, the downhole seismic sensors are multi-component sensors spaced along a borehole to form a subsurface sensing array. The array may, in some cases, extend the entire length of the borehole, though it is more common to have the array limited to the region of interest and the vicinity thereof (The sensors in this region generally exhibit the highest sensitivity to the desired formation parameters.)
The structure of the subsurface is commonly characterized as a series of substantially horizontal layers. This characterization is not a requirement, but it is a useful starting point for an initial velocity model. The initial velocity model, often derived from independent sources (e.g., experience, borehole logs, educated guesswork) or extracted from the data using classical methods, is iteratively refined until it converges on a useful representation of the actual subsurface structure. This refining is often done by comparing the measured seismic wave arrival times with those predicted using the velocity model, and adapting the model in ways that gradually reduce any mismatches. It is common to consider not only the mismatches in first arrival times of seismic waves (so-called “direct arrivals” because they have taken the most direct path), but also the mismatches in later arrivals which result from seismic waves taking more indirect routes, e.g., by reflecting from acoustic impedance mismatches between strata (formation layers).
To reduce the complexity of the solution process, the velocity model can, in some cases, be constrained. For example, the horizontal strata model assumption may be enforced, or the model may only be permitted to vary in the two-dimensional plane of the (inclined or deviated) borehole. Further, the velocity model may in some cases be limited to a fully isotropic velocity within each layer, though in the present disclosure the velocity of each layer is permitted to be anisotropic (transverse isotropy with a vertical symmetry axis).
With the foregoing context, traditional VSP surveys may fail to fully exploit the anisotropy information that can be derived from reflected arrivals. Efforts to improve the accuracy and efficiency of VSP surveys are ongoing.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.